Field of the Invention
The present embodiments relate to an image heating apparatus to be mounted on an image forming apparatus such as an electrophotographic copying machine and an electrophotographic printer.
Description of the Related Art
Conventionally, among image heating apparatuses (fixing apparatuses) used in electrophotographic-type image forming apparatuses, a belt heating type has been known (Japanese Patent Laid-Open No. 2013-114058). This apparatus includes a cylindrical fixing belt that rotates (hereinafter referred to as a belt). In addition, this apparatus includes a nip forming member, a stay, a heat source, and a belt guide member. The nip forming member is arranged inside the belt. The stay has high rigidity and presses the nip forming member. The heat source is arranged inside the stay. The belt guide member is arranged so as to cover the stay inside the belt.
A stay generally used for such an apparatus is formed of a sheet metal bent into a U shape in cross section, so that even with a thin sheet metal, the stay has rigidity against pressure applied to both ends thereof in the longitudinal direction. By bending the sheet metal, the second moment of area of the stay increases, and thus the stay obtains sufficient rigidity against the pressure direction. Therefore, it is necessary to secure the height of the stay in the pressure direction as much as possible.
The belt guide member is a resin member formed of heat-resistant resin. The belt guide member serves as a cover to protect, from the highly-heated stay, a temperature sensor, an electrical safety element, and wires thereof arranged inside the belt. The belt guide member also serves to guide the belt. With a portion of an outer peripheral surface of the belt guide member contacting an inner surface of the belt, the belt can rotate in a desired track.
Both ends of the belt guide member are pressurized against the stay by urging members, and thus brought into contact with an upper surface of the stay. In this way, a position where the belt guide member contacts the inner surface of the belt (hereinafter, referred to as a belt guide position) can be defined. Since a belt track in the vicinity of the nip forming member is particularly important for stabilizing the sheet conveyance and image quality, the belt guide member is often configured to contact the inner surface of the belt in the vicinity of the nip forming member.
In the image heating apparatus described above, however, the upper surface of the stay serving as a positional reference of the belt guide member is located away from the belt guide position. Therefore, such an image heating apparatus may have an issue that the belt guide position is easily changed by thermal expansion of the stay or belt guide member.
More specifically, in a case where the image heating apparatus is not sufficiently warmed up, such as at the time of cold start, the contact between the belt guide member and the fixing belt is weak. Therefore, the belt track is changed and the sheet conveyance becomes unstable accordingly. On the other hand, in a case where the image heating apparatus is warmed up, the contact between the belt guide member and the fixing belt becomes strong. Therefore, the rotary driving torque of the image heating apparatus tends to increase.
Therefore, it has been desired to stabilize the belt guide position in an image heating apparatus in which a heat source is arranged inside a stay and a portion of an outer peripheral surface of a belt guide member contacts an inner surface of a belt.